1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer, and a sheet transport apparatus capable of being provided in the image forming apparatus.
2. Related Background Art
In recent years, in an image forming apparatus, such as a copying machine and a laser beam printer using an electrophotographic process, for example, a toner image formed on a photosensitive drum provided in an image forming portion is transferred to a sheet supplied to the image forming portion by a sheet feeding portion, and the toner image is then fixed by a fixing portion. Among such conventional image forming apparatuses, there exists an apparatus in which a sheet transport path from the sheet feeding portion to the fixing portion is largely curved such that the size of the apparatus can be reduced.
FIG. 9 is a schematic view illustrating the structure of a laser beam printer which exemplifies such a conventional image forming apparatus.
In FIG. 9, reference numeral 101 designates a photosensitive drum serving as an image bearing member. Reference numeral 102 designates a charging roller serving as charging means. Reference numeral 103 designates a laser scanner serving as exposing means. The intensity of laser light is modulated by the laser scanner 103 based on an image information signal supplied from a host computer (not shown), and the modulated laser light is projected on the photosensitive drum 101, a surface of which is uniformly charged by the charging roller 102, such that an electrostatic latent image can be formed thereon.
The thus-formed electrostatic latent image is then carried to an opposing portion between a developing device 104 and the photosensitive drum 101 by the rotation of the photosensitive drum 101 in a direction indicated by an arrow, and is sequentially developed by the developing device 104. The toner image developed by the developing device 104 is then sequentially transferred to a sheet P, which is supplied to a transferring portion T from a pick-up roller 105, by a transferring roller 106.
The sheet P, to which the toner image is thus transferred, is separated from the photosensitive drum 101 in synchronous motion with the rotation of the photosensitive drum 101, and is fed to a fixing device 107. The sheet P is heated and pressed in the fixing device 107, and the toner image on the sheet is fixed as a permanent fixed image. Residual toner remaining on the photosensitive drum is collected by a cleaner container 118.
An apparatus of a heat roller system is conventionally used widely as the fixing device 107 for heating and fixing such an unfixed image (the toner image) on a recording surface of the sheet as the permanent fixed image. However, as disclosed in Japanese Patent Application Laid-Open No. H4-204980, a fixing device of a film heating system has been recently put into a practical use in the light of capability of quick start and reduction of energy.
As illustrated in FIG. 9, such a fixing device has, for example, a structure in which a heat resisting fixing film (or a fixing belt) 111 is sandwiched between a ceramic heater 110 serving as a heating member and a pressing roller 109 serving as a pressing member to establish a fixing nip portion, and when the sheet bearing the unfixed toner image to be fixed is introduced into the fixing nip portion between the fixing film 111 and the pressing roller 109, the sheet is nipped and conveyed together with the fixing film 111.
In such an event that the recording material is nipped and conveyed by the pressing roller 109 and the fixing film 111, heat of the ceramic heater 110 is transmitted to the recording material through the fixing film 111 in the fixing nip portion, and the unfixed toner image is thermally pressure-fixed onto the surface of the recording material by pressure applied in the fixing nip portion.
On the other hand, among conventional image forming apparatuses, there has been proposed an apparatus which is capable of forming an image on an envelope, as well as recording an image on a sheet P. An example of such an envelope is illustrated in FIG. 10A, and FIG. 10B which is a cross-sectional view taken along a line XB-XB in FIG. 10A. In FIGS. 10A and 10B, reference numeral 20 designates an envelope, reference numerals 21 and 22 designate a pair of structural pieces constructing front and back surfaces of the envelope 20, and reference numeral 23 designates a flap portion on which an adhesive for sealing is to be applied.
FIGS. 11A to 11C, and FIGS. 12A and 12B illustrate two kinds of envelopes. FIGS. 11A to 11C illustrate a COM10 envelope or a DL envelope which is generally used in Europe and United States, and in which the flap portion 23 (an opening portion) is provided on a longer side of a rectangle. On the other hand, FIGS. 12A and 12B illustrate an envelope which is widely used in Japan, and in which the flap portion 23 (an opening portion) is provided on a shorter side of a rectangle.
When the envelope undergoes printing (image formation) in the image forming apparatus, either envelope is generally inserted in the apparatus with its longer side of the rectangle being aligned to a longitudinal direction.
However, in a conventional image forming apparatus adapted to form an image on such an envelope, a wrinkle is likely to occur on the envelope, especially on the envelope of such a type as illustrated in FIGS. 11A to 11C, in the event that the envelope passes through the fixing device 107. The reason for occurrence of the envelope wrinkle will be described.
The envelope 20 is basically formed with a sheet of paper as illustrated in the development of FIG. 11C. In general, printing (image formation) is carried out on a front side of the envelope 20, and the envelope is transported with the front side (a print surface) being in contact with a heating side (a side of the fixing film) in the fixing device 107. Therefore, especially in a case of the envelope 20 containing much moisture, moisture of the surface being heated is chiefly evaporated, and this side is hence quickly shrunk in the fixing nip portion.
On the other hand, a back side of the envelope 20 is provided with a pasted overlap width portion as illustrated in the development of FIG. 11C, and the envelope is transported with the back side being in contact with a pressing side (a side of the pressing roller) in the fixing device 107. Accordingly, the back side is difficult to shrink in the fixing nip portion, as compared with the front side. In other words, when the envelope containing much moisture passes through the fixing device 107, its front side shrinks whereas its back side does not shrink, as illustrated in FIG. 13B.
Further, where the envelope 20 contains much moisture, the front side of the envelope 20 is likely to wave, and swelling portions is likely to unevenly appear as illustrated in FIG. 14A. Occurrence of swelling portions on the front side of the envelope 20 is accompanied by a fact that air is introduced into the envelope as illustrated in FIG. 14B.
In the event that the envelope 20 is passed through the fixing device 107 under such an air-introduced condition as illustrated in FIG. 15A, moisture in the swelling portion is evaporated in the fixing nip portion, and the swelling portion abruptly shrinks, leading to appearance of slight deformation in the envelope 20 as illustrated in FIG. 15B. During the passage of the envelope 20 as illustrated in FIG. 15C, the wrinkle hence appears from a starting point of a portion of that deformation to a trailing end of the envelope 20. To paraphrase the above, the envelope wrinkle is very likely to occur when air is introduced into the envelope at the time of fixation.
Turning back to FIG. 9, the sheet P fed out from the pick-up roller 105 is passed through a largely curved sheet transport path 120, and is fed to the transferring portion T. During the passage through the sheet transport path 120, the sheet P is conveyed while being curved upward along the sheet transport path 120.
Where the sheet P is an envelope which is comprised of a sheet of paper, the swelling portion occurs on its inner-side surface, which is a front surface on a folding side, due to the folding, as illustrated in FIG. 16A. FIG. 16B illustrates a folding angle. For example, in the case of the envelope 20 as illustrated in FIGS. 11A to 11C, a swelling portion slightly appears on its inner-side surface when the folding angle amounts to over 30 degrees. In such an event, the wrinkle occurs as discussed above during the passage of the envelope through the fixing device 107.
To paraphrase the above, the envelope wrinkle is likely to occur, where the envelope contains much moisture, and air is introduced into the envelope, and the swelling portion is present on the surface of the envelope when it reaches the fixing portion.